The ets family of genes encodes a novel class of transcription factors. Ets-1 is predominantly expressed in the lymphoid organs of neonatal and adult mice, whereas, ets-2 is expressed in every organ examined. In this study, the expression of ets-1 and ets-2 during murine embryonic development was investigated. Data show that ets-1 expression increases in embryos after implantation and during organogenesis, such that it is expressed in all the organs of 15-day embryos studied. In later fetal stages, ets-1 expression is predominant in the lymphoid tissues, brain, and organs that are undergoing branching morphogenesis (e.g., lung), but dramatically reduces in other organs, such as the stomach and intestine. In neonatal development, ets-1 is expressed only in the lymphoid organs and the brain. In situ hybridization analysis demonstrates that the expression of ets-1 occurs in mesenchymal cells of developing organs, in the nervous system, and in forming bone. Furthermore, the expression of ets-1 is upregulated in P19 cells induced to differentiate into mesoderm- like cells. Ets-2, on the other hand, is expressed in differentiated and undifferentiated P19 and F9 cells, and in all organs of embryonic, neonatal, and adult mice studied. These data suggest that ets-1 plays an important role in mesodermal cells associated with morphogenetic processes, such as organ formation and tissue-modelling, whereas ets-2 plays a more fundamental role in cells. To study the role of ets-2 during development, several new ets-2 founder mice were generated using ets-2 genes linked to the sheep metallothionein promoter.